1. Field of the Invention
This invention relates to a charged-particle beam writer used to write a microscopic pattern on a semiconductor wafer. In this type of apparatus, a pattern based on, for example, CAD (Computer-Aided Design) data is formed.
2. Description of the Related Art
In electron beam writers used in the process of fabricating semiconductor chips, it is important to increase the throughput. For this reason, a variable-shaped beam (VSB) strategy or a character projection strategy has been applied to this type of writer in recent years (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-237445). The variable-shaped beam strategy is to produce a beam whose cross section is rectangular, triangular, or of any other shape and draw a pattern on a semiconductor specimen with the produced beam. The character projection strategy is to prepare character data corresponding to a repetitive pattern on LSI (Large-Scale Integration) and draw a pattern on a semiconductor specimen by combining individual characters.
Use of the character projection strategy enables specific characters to be drawn in unison, which increases the drawing speed remarkably. The larger the character size becomes and the more the number of characters included in the drawn pattern is increased, the faster the drawing speed becomes. However, as the character size is made larger, the beam current becomes larger in proportion to the area. This causes a problem: the beam gets blurred on the specimen due to a space-charge effect or a so-called Boersch effect. To prevent the beam current from increasing, it is necessary to lower the current density much more than in the VSB strategy. Changing the current density requires to adjust the electronic optical system, which needs complicated, time-consuming work.
To overcome this problem, a scan-projection strategy has been proposed in recent years (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-217173). The scan-projection strategy is to scan a plurality of characters formed on an EB (Electron Beam) mask with an electron beam and draw a pattern on the semiconductor specimen. Use of this strategy enables a large pattern to be drawn on the surface of the specimen without a large increase or decrease in the current density. Furthermore, with this strategy, a character of any size can be formed on the EB mask and the drawing speed will not decrease significantly.
When a character is drawn with an electron beam, dimensional errors occur due to a proximity effect or such a phenomenon as fogging. In the scan-projection strategy, dimensional errors can be corrected by dividing each character into a plurality of elements, transferring them, and changing the irradiation time of the beam from one divided element to another. The effective dose of the electron beam applied to each character is influenced by its surrounding characters or the drawing pattern. That is, there is little sense in determining the dose for each divided element, taking only each character in account. Instead, taking into account comprehensively the positions and sizes of a plurality of characters arranged in the drawing pattern, it is necessary to determine the distribution of the dose for the scanning position on a character basis. At present, however, no consideration has been given to what dose distribution is given to which pattern on the specimen (that is, what dose is given to the scanning position).
As described above, in the existing techniques, measures to correct the dose of beam for a position on the surface of the specimen taking the character distribution on the pattern into account have not been adopted. Therefore, it is impossible to optimize the dose distribution for each character and, therefore, the drawing dimensions of each character are difficult to correct sufficiently. Consequently, there is a possibility that the drawing accuracy will decrease. This disadvantage may be encountered not only with an electron beam writer but also with an ion beam drawer.